Manager: We (meaning you) need to do task A. How long will it take?
Me: Task A will take X days to do.
Manager: That seems awful long.
Me: How long do you think it should take?
Manager: It surely could not take any longer than Y days.
Me: Ok, it seems you have an answer to your question then.
Later:
Manager: It’s been Y days, why isn’t task A done yet?
That pattern is also recommended in the book. Break down estimates into chunks of 5 hours or less.
With lots of smaller tasks, estimates tend to both be more accurate due to smaller scope, and some of the over/under inaccuracies will cancel out.